1. Field of Inventions
The present invention relates generally to medical devices that support one or more diagnostic or therapeutic elements in contact with body tissue.
2. Description of the Related Art
There are many instances where diagnostic and therapeutic elements must be inserted into the body. One instance involves the treatment of cardiac conditions such as atrial fibrillation and atrial flutter which lead to an unpleasant, irregular heart beat, called arrhythmia.
Normal sinus rhythm of the heart begins with the sinoatrial node (or "SA node") generating an electrical impulse. The impulse usually propagates uniformly across the right and left atria and the atrial septum to the atrioventricular node (or "AV node"). This propagation causes the atria to contract in an organized way to transport blood from the atria to the ventricles, and to provide timed stimulation of the ventricles. The AV node regulates the propagation delay to the atrioventricular bundle (or "HIS" bundle). This coordination of the electrical activity of the heart causes atrial systole during ventricular diastole. This, in turn, improves the mechanical function of the heart. Atrial fibrillation occurs when anatomical obstacles in the heart disrupt the normally uniform propagation of electrical impulses in the atria. These anatomical obstacles (called "conduction blocks") can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets, called "reentry circuits," disrupt the normally uniform activation of the left and right atria.
Because of a loss of atrioventricular synchrony, the people who suffer from atrial fibrillation and flutter also suffer the consequences of impaired hemodynamics and loss of cardiac efficiency. They are also at greater risk of stroke and other thromboembolic complications because of loss of effective contraction and atrial stasis.
One surgical method of treating atrial fibrillation by interrupting pathways for reentry circuits is the so-called "maze procedure" which relies on a prescribed pattern of incisions to anatomically create a convoluted path, or maze, for electrical propagation within the left and right atria. The incisions direct the electrical impulse from the SA node along a specified route through all regions of both atria, causing uniform contraction required for normal atrial transport function. The incisions finally direct the impulse to the AV node to activate the ventricles, restoring normal atrioventricular synchrony. The incisions are also carefully placed to interrupt the conduction routes of the most common reentry circuits. The maze procedure has been found very effective in curing atrial fibrillation. However, the maze procedure is technically difficult to do. It also requires open heart surgery and is very expensive.
Maze-like procedures have also been developed utilizing catheters which can form lesions on the endocardium (the lesions being 1 to 15 cm in length and of varying shape) to effectively create a maze for electrical conduction in a predetermined path. The formation of these lesions by soft tissue coagulation (also referred to as "ablation") can provide the same therapeutic benefits that the complex incision patterns that the surgical maze procedure presently provides, but without invasive, open heart surgery.
Catheters used to create lesions typically include a relatively long and relatively flexible body portion that has a soft tissue coagulation electrode on its distal end and/or a series of spaced tissue coagulation electrodes near the distal end. The portion of the catheter body portion that is inserted into the patient is typically from 23 to 55 inches in length and there may be another 8 to 15 inches, including a handle, outside the patient. The length and flexibility of the catheter body allow the catheter to be inserted into a main vein or artery (typically the femoral artery), directed into the interior of the heart, and then manipulated such that the ablation electrode contacts the tissue that is to be ablated. Fluoroscopic imaging is used to provide the physician with a visual indication of the location of the catheter.
In some instances, the proximal end of the catheter body is connected to a handle that includes steering controls. Exemplary catheters of this type are disclosed in U.S. Pat. No. 5,582,609. In other instances, the catheter body is inserted into the patient through a sheath and the distal portion of the catheter is bent into loop that extends outwardly from the sheath. This may be accomplished by pivotably securing the distal end of the catheter to the distal end of the sheath, as is illustrated in co-pending U.S. application Ser. No. 08/769,856, filed Dec. 19, 1996, and entitled "Loop Structures for Supporting Multiple Electrode Elements." The loop is formed as the catheter is pushed is the distal direction. The loop may also be formed by securing a pull wire to the distal end of the catheter that extends back through the sheath, as is illustrated in co-pending U.S. application Ser. No. 08/960,902, filed Oct. 30, 1997, and entitled, "Catheter Distal Assembly With Pull Wires," which is incorporated herein by reference. Loop catheters are advantageous in that they tend to conform to different tissue contours and geometries and provide intimate contact between the spaced tissue coagulation electrodes (or other diagnostic or therapeutic elements) and the tissue.
One type of lesion that has proven to be difficult to form with conventional loop catheters is the so-called "flutter lesion" that is used to cure atrial flutter and also forms part of the curative lesion pattern in some instances of atrial fibrillation. The flutter lesion, which blocks a re-entrant pathway that encircles the tricuspid annulus, extends from the inferior vena cava to the tricuspid annulus along the isthmus region of the heart. This area includes a mass of tissue known as the Eustachian ridge. The irregular geometry of the tissue in this area often prevents conventional loop catheters from achieving the level of tissue contact necessary to form a curative lesion. Thus, the flutter lesion must be formed by dragging the tip electrode of a steerable catheter over the target region while the steering mechanism deflects the tip electrode into contact with the tissue. This process must be repeated many times to form a complete lesion with bi-directional block. As a result, and despite the fact that the flutter lesion is relatively short (at 1-2 cm, it is the shortest lesion in the set that cures atrial fibrillation), its formation takes a relatively long time. The flutter lesion typically takes 30 to 60 minutes to complete. To put that in perspective, the lesion that isolates the pulmonary vein, which is about 15 cm in length, also takes about the 30 to 60 minutes to complete.
Accordingly, the inventors herein have determined that a need exists for loop structures that can be used to create lesions within bodily regions, such as the isthmus region of the heart, which have an irregular geometry. In particular, the inventors herein have determined that a need exists for loop structures that can exert enough force on the tissue to form a curative lesion in tissue that has an irregular geometry, such as tissue within the isthmus region of the heart.